This research project is designed specifically to study the dynamic determinants of left ventricular filling under normal and pathological conditions using the new technique of transmitral flow measurement by a flow transducer inserted above the mitral valve. When combined with the measurement of other hemodynamic parameters (aortic flow, intracardiac pressures, electrograms, and echogram of the mitral valve) this approach yields valuable information on the physiology of atrio-ventricular function and dynamics. The experimental approach is designed to investigate on a beat-to-beat basis, the following determinants of left ventricular filling: 1) phasic left ventricular relaxation rate; 2) left ventricular dynamic diastolic compliance; 3) mitral valve motion; 4) contribution of atrial systole; 5) atrial reservoir function. These five areas will be investigated under diverse physiological conditions of heart rate, P-R intervals, and functional state of the heart; and under four pathological conditions: 1) mitral regurgitation; 2) mitral valve stenosis; 3) mitral annulus stenosis; and 4) abnormalities of rhythm. With this unique approach we anticipate that our results might offer definitive solutions to several significant problems related to ventricular filling and subsequent cardiac function which have heretofore been approached only by indirect and inferential means. Elucidation of atrioventricular dynamics in a way not possible by other means should also permit a more meaningful interpretation of the results of cardiac catheterization and non-invasive clinical methods such as echograms.